Optical communication systems employing optical fibers have been rapidly put into practice because of the large capacity of information which can be transmitted through it at a very high speed. Specially for inter-continental transmission, optical fibers were used in building undersea networks already few decades ago. But the ever growing popularity of telecommunications e.g. on the Internet and a price dumping for telecommunications due to a liberalization of this economical sector end up of a need to increase substantially the telecommunications transmission possibilities without implying to high cost. The use of a technique called wavelength division multiplexing (WDM) shall be very promising since it permits to transmit in already lay down optical fibers, optical signals at multiple different wavelengths (more then 50 in a same optical fiber). As before, it is necessary to use from time to time (e.g. every 100 km depending on several parameters) amplifiers like transponders for those optical signals. Many amplifiers will have to be used now for a high number of different wavelengths. These amplifiers are then concatenated which harden the condition of an acceptable spectral bandwidth.
To verify such condition, a spectral equalization must be performed on the different transmitted wavelengths. When only few different wavelengths are used a passive spectral equalization may be sufficient. In U.S. Pat. No. 6,084,695 is disclosed an example of passive spectral equalization based on the use of a filter like a Fabry-Perot filter, little selective (or Fizeau filter). Such filter is used in combination of a multiplexer while different wavelengths are transmitted through different input fibers. Each of these input fibers have then their ends located on an end plane. The multiplexer comprises a dispersing element or grating, a collimating optical element, a reflector system and produces an output beam collected by an output fiber. The reflector system is an adjustment element whose orientation enables the centering of the luminous beams with respect to the wavelengths considered on the elementary pass bands. The spectral equalization is performed here in order to bring each of these pass bands having a maximum closer to a rectangular shape. The filter is additionally placed to act on each of them such that the period of its transmission spectrum is equal to that of the central wavelengths of the elementary bands of the multiplexer. This filter can be accommodated in a superimposition region of the different wavelengths and will act on each of them in a same way.
However, as wavelength division multiplexed optical transmission systems begin to be deployed commercially, the need for active management of spectral gain is increasingly important. Indeed, individual channel powers comprising at least a respective different wavelength may vary over time. Furthermore, the gain spectrum varies with dynamical load. In WO98/06192 is disclosed a technique for active management of the spectral gain, based on a polarization-insensitive diffractive ferroelectric liquid crystal (FLC) in-line filter. It is made of a reconfigurable holographic filter arranged along an optical path between the optical inputs and the optical outputs. The equalization is based here on a spatial deflection such that the hologram will reshape the propagated beam correspondingly. The holographic filter comprises a FLC pixellated spatial light modulator (SLM) displaying dynamic holograms in conjunction with a fixed binary-phase high spatial frequency grating. The reconfiguration of the holographic filter is achieved in combination with processing means storing data on a number of predetermined holograms. Latter provide signal power equalization for a number of optical signals of predetermined different wavelengths. The active management disclosed in WO98/06192 is limited to these predetermined holograms defined for predetermined wavelengths. It cannot be successfully applied when a shift or a change of few or all of the wavelengths occurs. But this is just what becomes increasingly important with the actual high number of different wavelengths used simultaneously for the transmission of optical signals under WDM.
It is an object of the present invention to provide a dynamic spectral equalizer with a high dispersive capacity and applicable for optical signals made of a variable number of different wavelengths while being almost polarization and temperature independent.
According to a first aspect of this invention, this object is attained by a spatial light modulator comprising a planar cell filled with a liquid crystal based substance having a variable scattering property with respect to an electric field, and at least two electrodes enclosing said planar cell from each side of its plane to apply a voltage on said liquid crystal, said applied voltage being non-uniform in space implying an electric field inside said cell with a variable spatial distribution along said plane adapted for the modulation of at least a pass band comprising at least a wavelength at which an optical signal is transmitted through said cell, while said modulation taking the form of a variable attenuation.
According to a second aspect of the invention, its object is achieved by a dynamic spectral equalizer comprising an optical input for an incident light beam consisting of optical signals transmitted via at least a wavelength, an optical output for collecting a resulting light beam, said dynamic spectral equalizer comprising further a spatial light modulator, in accordance with the first aspect of this invention.
Furthermore, its object is also achieved by a telecommunications device incorporating a dynamic spectral equalizer in accordance with the second aspect of the invention.
It is taken advantage of the use of a spatial light modulator (SLM) made of a cell filled with a liquid crystal (LC) based substance having a variable scattering property with respect to the amplitude of an electric field present in that cell. In a paper from K. Takizawa et al., Applied Optics, Vol. 37, pp 3181-3189, 1998, is disclosed a polarization independent optical fiber modulator made of polymer-dispersed liquid crystals (PDLC). This PDLC materials has LC droplets of several micrometers or less in diameter dispersed within the polymer. By controlling the difference between the refractive indices of the LC droplets and the surrounding polymer, the state of the PDLC can be varied continuously from opaque to transparent. When no electric field is applied to this modulator, the LC molecules face different directions for each droplet. An optical beam that is transmitted through said modulator, will be strongly scattered. This reduces substantially the power of light that propagates, causing the modulator to be in an off state. Applying an adequately large electric field to the PDLC causes the LC molecules to be arranged in the direction of the electric field. Then, both the polymer and the LC droplets show nearly the same refractive index. In this case, the optical beam that propagates through said modulator is straight without scatter. Since such modulator is based on the light-scattering effect of the PDLC, it can be used to modulate an optical beam without depending on polarization.
In the present invention it is now a non uniform electric field which is applied on a cell filled with a similar LC based substance. In such a way, it is then take advantage of the variable scattering property (loss) of said substance to build a variable attenuator. Latter will then be adapted for a shaping of the transmission loss of a pass band comprising at least a wavelength at which an optical signal is transmitted through said cell e.g. by flattening or equalizing said pass band. By using a plurality of electrodes in the direct vicinity of said cell, it is then possible to build an electric field inside it which will permit advantageously to modulate a continuous spectrum from optical signals transmitted through said cell. For that, the different pass bands present in said continuous spectrum while at least few of them comprise at least a respective different wavelength will have to be transmitted through said cell at different regions. Latter will correspond to different values of the amplitude of scattering such to be adapted to modulate the pass bands of different wavelengths.
Such SLM is favorably used to build a dynamic spectral equalizer according to the present invention. Latter contains a spectral dispersive element placed on an optical path of an incident light beam. This spectral dispersive element will advantageously spread continuously said incident light beam onto said SLM such that at least each different wavelength present in said incident light beam are focused or imaged towards a different spatial region of said SLM. Since the plurality of electrodes with which an electric field is applied to the cell of said SLM can be controlled independently, it will be possible to adopt in real time the modulation of the continuous spectrum to a shift or even a drop of some or addition of few new wavelengths present in the light beam.
Further advantageous features of the present invention are defined in the dependent claims and will become apparent from the following description and the drawing.